IE851551L

IE851551L - Non-contact shaft angle detector

Info

Publication number: IE851551L
Application number: IE851551A
Authority: IE
Original assignee: Energy Innovations
Priority date: 1984-06-25
Filing date: 1985-06-21
Publication date: 1985-12-25
Also published as: ZA854600B; NO852532L; DK284885A; IL75604A0; NO169865C; AU569686B2; GB2162635A8; JPS6114518A; IE56671B1; DK284885D0; AU4380685A; DE3579767D1; EP0169657A2; GB8515358D0; DK162908B; EP0169657B1; GB2162635A; DK162908C; EP0169657A3; US4587513A

Abstract

Apparatus and method for non-contact determination of shaft angle utilizes a patterned disc secured to the shaft or other rotor being observed. The disc has sectors of different optical properties whereby radiant energy directed to the disc will be encoded by the sectored pattern. A set of detectors, which each correspond to one of the sectors, provides variable output signals according to the amount of radiant energy attenuated by the disc and directed onto the detectors. These signals are converted to a set of digital signals which define a unique rotational position, and a computing means calculates angular position from these signals. The computing means will, if necessary, compare the digital output signals and then calculate angular position in a manner which is insensitive to small decenters (or lack of concentricity) in the arrangement of the shaft, disc, detectors, and any associated optical system. [US4587513A]

Description

►SS67 1 -1- MOBCOMTACT SHAFT AMGLE DETECTOR This invention relates to apparatus and method foe noncontact optical measurement of she angular position of a 5 shaft or like element supported to sotate about a predetermined ait is. Various types of mechanism are known for this general purpose, but they have limitations due to concentricity and resolution requirements, complexity, expense, or reliability. 10 Typical of prior art devices are drums or discs affixed to the shaft in question and carrying Magnetic or optical "marks'*' which define some increment of shaft angle. Higher resolution requires la larger number of marks, and as this number increases it becomes necessary to either 15 increase the radius of the drum or disc to maintain a readable separation of the marks, or to make the marks smaller and the construction of the optical or magnetic readouts more precise. Some devices use a single circle of such marks together with some index or "0 angle" indicator; 20 the detecting mechanism simply counts the number of marks as the shaft rotates from zero position to determine the shaft angle. An interruption of device pov;er may cause the counting system to lose track of the total count, or angle. -2- &n absolute angle indication can be obtained if each mask is replaced by a unique code uoed. Howevere the sj.se of such code words determines the resolution, and the larger the word, the wore constraints are placed upon construction 5 tolerances, leading to saore complexity„ closer tolerances, end more expensive devices. &n optical rotor rotation sensing system for reading out power consuraption values feora a wetthour meter is disclosed in U.S. Patent Bo. 4,327,362 issued 27 kpril 10 1982 to Robert J. Boss, this system provides the rotor of the meter with a surface which is light reflective over 180° and light sbsorbtive over the remainder. Light froa an LSD driven from en AC signal is directed to this surface of the rotor through an optical fibre cable and reflected light is 15 transmitted through another optical fibre cable to a photodetector. The resultant pulses are counted and stored in the counter for later transmission to a remote monitoring site. f»n angle-position transducer system of a direct 20 reading analog-to-digital type is disclosed in U°S. Patent No. 4,320,203 issued IS Match 1982 so Harold Gursts&y.

There a source of light which provides a thin "lina™ of light, preferably from a laser source, is directed transversely to a transparent angle-shaped opening arranged ground the suKface of a drum cersistl on the shaft being monitored. A photodetaceor mounted on the opposite side of the drum from the light source secoives a variable amount o light according so the sheSt notation, and the resulting 5 variable voltage signal is converted to a digital signal which is used to drive a digital indicator.

U.S. Patent Ho. 3,918,814 issued 11 November 1975 to Sidney Vgeiser, discloses an optical position sensor in which a beam of light is collimated and directed by an 10 optical fibre cable through the center of a four quadrant photodetector (quad detector), through a lens End onto a target having a regular target area of uniform reflectivity Reflected light returns though the lens to the quad detector, and the resultant output voltage from each 15 quadrant bears a direct relationship to the displacement of the target image along either the * ot j axis, while £ ait is measurements can be achieved with a more complex detector and circuit. However there is no provision for determining displacement in rotation, and the required uniform 20 reflectivity of the target area will preclude such a measurement.

In one aspect the invention provides a noncontact shaft angle detector comprising a pattern of sectors on a shaft, the angular position of which is to be determined, said sectors having different optical properties whereby radiant energy directed to said pattern will be encoded by the sectors, a set of detectors arranged to receive radiant energy from said pattern, each having an output connection which provides a variable output signal in dependence upon the amount of the radiant energy directed thereto from said sectors according to the angular position of said pattern, optical means for directing the encoded radiant energy from said pattern onto said detectors, and analog to digital converting means receiving separate signals from each of said output connections and providing a set of separate digital output signals uhich together define a unique rotational position of said pattern., In another aspect the invention provides a method of determining the angular position of a rotatable member about its axis of rotation, comprising the steps of providing on the member a pattern substantially concentric with said axis, said pattern being divided into - 5 - sectors of different radiation attenuating capability, directing radiant energy onto the surface of the pattern, sensing the attenuated radiation from the pattern 5 over a plurality of discrete areas so as to generate analog signals proportional to the attenuated radiant energy impinging upon each said area in dependence upon the angular position of said pattern, converting each of said analog signals into digital 10 signals, and calculating from said digital signals the angular position of the member.

The pattern may, for example, be printed onto the end of the shaft, or onto a small disc which is then attached 15 to the shaft.

The pattern can be illuminated by either ambient light or an artificial radiation source.

In one embodiment the pattern is reflective, but the pattern could for example, comprise of areas of differing 20 optical transmission and be backlit.

One simple embodiment of this invention utilises a circular pattern which is divided into equal sized semi circular areas of high and low reflectivity, and a "quad" detector which is comprised of 4 equal size quadrants of a circle. The responsivity of the detector quadrants can be adjusted in different wavelength regions to improve the contrast of the target pattern, or to reject contaminating radiation sources.

For example, in some applications visible radiation from the sun or flickering fluorescent tubes can perturb the detector readings. In these cases an infrared filter in front of the detector will eliminate most of this clutter radiation, and still pass the radiation produced by an infrared LED. The LED output can be pulsed to further discriminate its radiation from natural sources.

In constructing apparatus according to the invention the concentricity of the pattern and the center of the quad detector may be easily controlled, and "off axis" reading may not present any particular problem. However, in those applications where such concentricity is difficult to control, the invention includes a preferred feature which reduces error introduced by such lack of concentricity. This feature is described in connection with the readout and analysis of the signals from the segments of the quad detector.

In a preferred embodiment, radiant energy attenuated by the different parts of the disc is directed or focused onto the four elements of the quad detector by a simple optical system. In addition to sharpening the image I observed by the detector, and collecting more radiation, this also allows enlargement or reduction of the image for applications where a very small or very large disc might be necessary because of size or space limitations. Each segment of the quad detector is an independent photodetector having an output which is proportional to the amount of radiant energy to which it is exposed within its wavelength region of responsivity. Each of these output signals is directed to an appropriate electronic amplifier, and the amplifier outputs are connected into analog or digital converters.

At the converter outputs of this preferred embodiment there are, accordingly, a set of separate digital output signals which define the angular position of the disc, and therefore of the shaft to which the disc is attached.

These digital output signals are directed to a processor device, preferably a microprocessor. The microprocessor - 8 - in turn calculates the shaft angle which corresponds to that particular set of digital signals.

In the event exact concentricity of the shaft axis, the disc, the optical system and the quad detector center , 5 is not readily attainable, in accordance with a preferred feature of the invention the microprocessor angle 4 computation algorithm can compensate for modest amounts of decenter. This capability is of particular value in situations where the position of the shaft axis may shift lO during rotation, or where it is costly to achieve concentricity of the disc on the shaft, or of the shaft/disc assembly to the quad detector and optical system assembly.

A preferred embodiment of the invention is described 15 below by way of example only with reference to the accompanying drawings of which: Figure 1 is a schematic drawing of an apparatus provided in accordance with the invention; and Figure 2 is a circuit diagram shoeing details of one 20 segment of the quad detector, its power supply and its output amplifier, and a controllable LED source of radiant energy. 4 - 9 - Referring to Fig. 1, a rotstable member is represented by the shaft 10 which is supported for rotation about an axis 12. It is desired to determine accurately the angular position of this shaft. Xn accordance with the 5 invention a small disc 15 is fixed to the shaft 10. The disc pattern is divided in half, as shown, by having two separate areas ISA and 15B of different optical properties, such that each will attenuate radiant energy directed thereon (or through) in a distinctly different manner. This 10 may be achieved in any suitable manner, as by constructing the disc of different halves, or appropriately coating its surface, to obtain the desired result. In one succesful embodiment constructed according to the invention, the disc is provided with suitable coatings which make one half of 15 its surface reflective and the other half absorptive. It should be noted that each of the different halves of the disc covers two quadrants of the disc surface.

The disc is described cs "fixed" to the member or shaft under observation, but it should be understood that 20 such fixation may be of a temporary nature, for example when using the invention in testing or assembling operations. However, the fixing of the disc to the shaft, even if temporary, is tight enough that the two rotate together. The - lO - surface o£ the disc is uniformly flooded with radiant energy of desired wavelength. In some uses, this may simply be ambient light (daylight or artificial) if such wavelengths are satisfactory. Where it is desired to minimise optical S interference from ambient light, it is useful to utilise a source of r&diant energy in the invisible part of the spectrum, e.g. infrared light, and this is illustrated in Fig. 1 as an infrared LED (light emitting diode) 16 which is connected to a suitable power source via an electronic 10 switch 18. Details of a suitable circuit are shown in Fig. 2. The trigger input to the switch 18 provides a way to control or "strobe" the light output of the LED for timing purposes and to discriminate the LED output from slowly varying natural radiation sources. 15 The radiant energy from the LED light source is reflected differently by the two different parts ISA and 15B of the disc, and the encoded energy is directed through a simple lens system 20 which focuses an image (inverted and reversed) onto the quad detector 22. This is a commercially 20 available device which is available with different response characteristics, in this case being responsive to a range of intensity of infrared light as emitted from the LED.

Detector 22 is comprised of four photodetector elements Ql, 02, Q3, and Q4, each of which has a distinct output signal line 24A, 24B, 24C, and 24D which lead to individual electronic amplifiers 25A, 25B, 25C, and 25D. The power supply and amplifier circuit for one element Q4 is shown in Fig.2. The amplified output signals thus are an analog 5 representation of the quantity of radiant energy directed to the respective quadrant elements of the sensor 22.

The outputs of each amplifier are connected to the inputs of conventional analog-to-digital (A to D) converter circuits 28A, 28B, 28C, and 28D which generate four digital 10 outputs on their outputs 29A, 29B, 29C, and 2SD. This group or set of digital words defines a specific angular position of disc 15, and thus shaft 10, with respect to the fixed position of sensor 22. Expressed another way, in the illustration angle 0° is a vector from the center of the 15 detector extending between the segments Q1 and Q2 and the corresponding shaft angle 0" locates the disc with the line between the sections 15A and 15B extending horizontally and the section 15B at the top, wheseby there is maximum illumination of the segments Q1 and Q2 and minimum 20 illumination of segments Q3 and Q4.

A set of digital words is thus transmitted for each reading to apparatus for converting this information into an angular representation or expression. A preferred apparatus for thjrs purpose is a microcomputer 30. In one successful - 12 - embodiment of the invention fa Commodore 64 (Til) unit with a 6502 microprocessor is used. The angular representation which it calculates can be used to drive a conventional display device 32, and/or the representation can be recorded 5 by a suitable printer 34„ Assuming the location of angle 0° as previously explained, the output signals from segments Q1 and Q2 will be the maximum, i.e. corresponding to full radiant energy thereon, and the output signals from segments Q3 and Q4 will 10 be minimum, i.e. the least amount of reflection possible; note that this assumes an inversion and a side-to-side reversal of the radiant light image by the lens. It follows that, proceeding clockwise as the sensor is shown in Fig. 1, angle 90° will have Ql and Q4 at full or maximum signal, at 15 180° Q4 and Q3 will be at full signal, and at 270° Q3 and Q2 will be at full signal.

The signal outputs SI, S2, S3 and S4 from the four quadrants of the detector can be translated into an angle by using the following sequence of mathematical operations 20 (uniform illumination of the pattern and identical sesitivity of each detector quadrant is assumed for simplicity) . - 13 - First, form the intermediate sums: Al = SI + S4 A2 = S2 * SI A3 - S3 + S2 5 A4 - S4 * S3 SUM = 2 (SI + S2 S3 + S4) Next, locate the quadrant Q which contains a line bisecting the image of the pattern into two equal halves and that is an illuminated quadrant: 10 If A2>A4 and A3 < Al, then Q « 1 If A2 > A4 and A3 > Al, then Q » 2 If A2 < A4 and A3 > Al, then Q = 3 If A2 < A4 and A3 < Al, then Q = 4 Finally, find the angle using the correct equation 15 for the proper quadrant: Quadrant 1 (0=1) Angle =» (SUM/4-S2-S1) • 90° (S3 - SI) Q - 2 20 Angle - (SUM/4-S3-S2) ° 90° + 90° (S4-S2) Q s 3 Angle = (SUM/4-S4-S3) " 90° •=■ 180° (S1-S3) - 14 - Q - < angle - (SUM/4-S1-S4) ° 90° * 270° (S2«S4) For example, if; 5 SI = 70 S2 = 100 S3 - 30 S« ° 0 i then: Al =70 A2 = 170 A3 = 130 A4 » 30 J SUM - 400 Since A2 "> M and A3 Al, then Q = 2 and the 10 bisector pointer is located in the second quadrant.

The angle is therefore: Angle = (100-30-100) ° 90 + 90° -100 =117° 15 This analysis technique is insensitive to small decanters of the pattern image on the quad detector since it finds the bisector of the energy distribution.

Following are the program listings in Microsoft Basic for the Commodore 34 microcomputer to determine 20 degrees of rotation of the disk: 4010 REM * READ ENCODER * 4020 REM 4025 FOR J=1 TO 2 : RR=0 : FOR P= 1 TO 4 STEP 1 4 - 15 - 4030 POKE EN+P-1,0 :Q(P)«PEEK(EM) sNEXT 404 5 FOR P-1T04 :IF ABS (QW (P) =Q (P) ) > SR THEN RR»l 4055 NEXT : FOR P=lT04 :QW(P)=>Q(P) sNEXT 4060 NEXT J :IF RR=1 THEM 4025: REM WAIT FOR STABLE READING 5 4070 F0RP=1T04 :Q (P) " (MO-Q(P)) /H (P) :NE)IT 4080 GOSUB4SOO :AH=360-AN 4100 : IF WRK <>0 THEN AI=AN :RO=0 : RETURN :REM STORE INIT ON ODD PASS 4110 RO=AN-AI : IF R0<0 THEN RO=R04-360 10 4120 : IF RO < 70 THEN RO=RO-s-360 4200 RETURN 4500 REM 4510 REM * CALC. ANGLE ® 4530 15 M(1) =Q(1) 4-Q(4) :M(2)"Q(2)-pQ(l) :M(3)-=Q(3) 4-Q(2) :M(4>-=Q(4) *Q(3) 4540 SUM" (Q(X) -S-Q(2) -t-Q(3) +Q (4)) /2 4550 : IF M(4) > M(2) THEN 4580 4560 : IF M(3) > -M(l) THEN Q=2 :GOT04600 4570 Q=1 :GOTO 4600 20 4580 : IF M(3) > =M(1) THEN Q=3 :GOT04600 4590 Q=4 4600 ON Q GOTO 4900,4910,4920,4930 4900 AN=(SUM-Q(2)-Q(m/(Q(3)-Q(ll) "90 -.RETURN 4910 AN=(SUM-Q(3)-Q(2))/(Q(4)-Q(2)) *90 +90:RETURN - 16 - 4920 AN=(SUM"Q(«)»Q(3))/(e(l)"0(3)) ">90 +180:RETURN 4930 AN=(SUM-Q(l)-Q(4))/(Q(2)-Q(4)) ®90 *270:R£TURN 5000 REM Steps 4000 through 4500 read the A/D outputs, 5 execute the comparisons for stability and paefoera the centering calculations? If these are not required in a particular installation, they may be deleted. Steps 4510 through 5000 perform the calculation of the angular representations. 10 Hhile the method herein described, and the form of apparatus for carrying this method into effect, constitute preferred embodiments of this invention, it is to be understood that the invention is not liwited to this precise method and form of apparatus, and that changes may be made 15 in either without departing from the scope of the invention, which is defined in the appended claims. -17 -

Claims

CLAIMS:

1. „ A noncontact shaft angle detector comprising a pattern of sectors on a shaft, the angular position of which is to be determined, said sectors having different 5 optical properties whereby radiant energy directed to said pattern will be encoded by the sectors, a set of detectors arranged to receive radiant energy from said pattern each having an output connection which provides a variable output signal in dependence upon the 10 amount of the radiant energy directed thereto from said sectors according to the angular position of said pattern, optical means for directing the encoded radiant energy from said pattern onto said detectors, and analog to digital converting means receiving separate 15 signals from each of said output connections and providing a set of separate digital output signals which together define a unique rotational position of said pattern.

2. A shaft angle detector as claimed in Claim 1, further comprising computing means arranged to use said digital 20 output signals from said converting means to calculate the angular position of said pattern by comparative analysis of the magnitude of each digital output signal of a set.

3. A shaft angle detector as claimed in Claim 2, wherein said computing means is arranged to compare the digital 25 output signals of a set and then calculate the angular - 18 - position of said pattern in such a manner as to be insensitive to small decenters in the pattern/optical system/detector arrangement„

4. A shaft angle detector as claimed in any preceding claim, said sectors having reflective surfaces of different reflective properties, a selectively actuatable radiation source located to direct radiation onto the entire area of said pattern, and means connected to actuate said radiation source when it is desired to read the position of said shaft.

5. A shaft angle detector as claimed in Claim 4, wherein said radiation source provides radiant energy in a predetermined invisible band of the spectrum.

6. A shaft angle detector as claimed in Claim 4 or Claim 5 wherein said radiation source is arranged to generate regular pulses of radiation.

7. A shaft angle detector as claimed in any preceding claim wherein means are provided for forming an image of said pattern on said set of detectors.

8. A shaft angle detector as claimed in any preceding claim wherein said set of detectors comprises a quadrant detector.

9. A method of determining the angular position of a rotatable member about its axis of rotation, comprising the steps of - 19 - providing on the member a pattern substantially concentric with said axis, said pattern being divided into sectors of different radiation attenuating capability, directing radiant energy onto the surface of the 5 pattern, sensing the attenuated radiation from the pattern * over a plurality of discrete areas so as to generate & analog signals proportional to the attenuated radiant energy impinging upon each said area in dependence upon 10 the angular position of said pattern, converting each of said analog signals into digital signals, and calculating from said digital signals the angular position of the member. 15

10. A method as claimed in Claim 9, wherein the digital signals are processed in a manner which avoids sensitivity to disparity in concentricities in the pattern/sensor system.

11. A method as claimed in Claim 9 or Claim 10 wherein 20 said areas are four in number.

12. A method as claimed in any of Claims 9 to 11 wherein said radiant energy is generated by an artificial light source.

13. A method as claimed in any of Claims 9 to 12 wherein 25 said radiant energy is directed onto the entire surface of V - 20 - said pattern.

14. A method as claimed in Claim 12 or Claim 13 wherein said radiant energy from said light source is pulsed.

15. A method as claimed in any of Claims 12 to 14 wherein 5 said radiant energy is invisible.

16. A method of determining the angular position of a rotatable member, substantially as described with reference to the accompanying drawings„

17. A shaft angle detector substantially as described 10 with reference to and as shown in the accompanying drawings. MACLACHLAH & DOHALDSOW Applicants' Agents 47 Merrion Square DUBLIN 2. ■j 4